1. Field of the Invention
The present invention relates to an electric power steering apparatus for detecting steering torque on the basis of a difference between rotating angles of first and second resolvers which are located at both ends of a torsion bar connected to a steering wheel, and for assisting a steering operation by controlling a motor in accordance with assist amount determined on the basis of the steering torque.
2. Description of the Related Art
Conventionally, an electric power steering apparatus, which reduces steering force through a steering wheel by providing a steering mechanism connected to a steering shaft with assist force by a motor, has been known. In the conventional electric power steering apparatus, a torque sensor using a torsion bar may be employed for detecting a steering torque. Specifically, the torsion bar with a pair of rotating angle sensors is disposed between the steering shaft and a steering gear box. Torsion of the torsion bar, i.e. the steering torque, is calculated from difference between rotating angles detected by first and second rotating angle sensors. As the rotating angle sensors for the torque sensor, resolvers can be employed because of its high mechanical reliability. The electric power steering apparatus having the aforementioned structure is described in Japanese Patent Laid-Open Publication No. 11-321689.
In the torque sensor having a pair of resolvers disposed at the both ends of the torsion bar, a malfunction of the sensor is detected in such a way that described in Japanese Patent Laid-Open Publication No. 8-289521 or No. 8-210874. The aforementioned way for detecting the malfunction depends on that the sum of the square of the amplitude of the Sin signal and the amplitude of the Cos signal, which are output signal from the resolver, is a constant value (A*Sin2+A*Cos2=c, ‘A’ and ‘c’ mean constant value). Therefore, when the sum (A*Sin2+A*Cos2) of the squares exceeds predetermined threshold value, it is judged that malfunction occurs.
However, the detection of the torsion of the torsion bar using a pair of resolver needs complicated calculations. Therefore, the calculations for obtaining the steering torque and the steering angle are also complicated.
Further, since the resolver including winding coils is a kind of the electrical transformer, amplitudes of Sin phase signal and Cos phase signal vary in accordance with temperature variation. Therefore, a threshold value for aforementioned malfunction judging is needed to set at a value that temperature variation offset value T is added to the C (constant value). The temperature variation offset value T may set to be relatively large value to avert errors in detection caused by temperature variation. Accordingly, it is difficult to detect the malfunction immediately, and accuracy of the malfunction judging is deteriorated by temperature variation.